Master thesis
Universiteit Utrecht
In collaboration with Witteveen+Bos
The potential of
phytostabilization on
environmental and human risk
reduction in mine waste areas
i
ii
The potential of
phytostabilization on
environmental and human risk
reduction in mine waste areas
The research reported in this document has been carried out by
Kelly (K.) Goris B.Sc.
email: k.goris@students.uu.nl
Under the joint supervision of
Ingrid (I.) Rijk M.Sc.
email: i.rijk@witteveenbos.nl
Witteveen+Bos Consultancy & Engineering
Van Twickelostraat 2, 7400 AE Deventer, The Netherlands
Dr. Bas (B.J.) Blaauboer
email: b.blaauboer@uu.nl
Institute for Risk Assessment Sciences (IRAS), Div. of Toxicology
Universiteit Utrecht, Yalelaan 104, 3508 TD Utrecht, The Netherlands
Utrecht, May 2012 – July 2012
iii
Table of Contents
Summary.................................................................................................................................... 1
Chapter 1.
Introduction ........................................................................................................ 3
Chapter 2.
Risks associated with mining sites .................................................................... 5
Mining related pollution ................................................................................................................................... 6
Dispersion and exposure pathways .............................................................................................................. 7
Risk receptors ........................................................................................................................................................ 8
Chapter 3.
Plant - Environment interactions ...................................................................... 9
Plant – physical environment interaction.................................................................................................... 9
Plant – microbe interaction ............................................................................................................................ 12
Soil amendments ............................................................................................................................................... 13
Chapter 4.
Plant – Community interactions on risk reduction........................................ 14
Requirements for tailing-resistant vegetation ........................................................................................ 14
Characteristics of risk reducing vegetation .............................................................................................. 15
Climate dependant ............................................................................................................................................ 16
Plant selection ..................................................................................................................................................... 17
Genetic engineering.......................................................................................................................................... 17
Crops of economic value ................................................................................................................................ 18
Chapter 5.
Results revegetation studies ............................................................................ 18
Surface runoff ...................................................................................................................................................... 18
Leaching ................................................................................................................................................................ 19
Bioavailability ..................................................................................... Ошибка! Закладка не определена.
Plant establishment ........................................................................................................................................... 18
Chapter 6.
Discussion & Conclusion ................................................................................. 22
Plant interactions ............................................................................. Ошибка! Закладка не определена.
Success factors for phytostabilization ........................................................................................................ 22
iv
Results field studies........................................................................................................................................... 23
Applicability........................................................................................ Ошибка! Закладка не определена.
References ............................................................................................................................... 26
v
Abbreviations
AF
Al
AMD
As
B
BF
Ca
Cd
Cu
DOC
Fe
Hg
K
Mg
Mn
Ni
P
Pb
PGPB
PM
S
Se
TF
Zn
Accumulation Factor
Aluminium
Acid Mine Drainage
Arsenic
Boron
Bioconcentration Factor
Calcium
Cadmium
Copper
Dissolved Organic Carbon
Iron
Mercury
Potassium
Magnesium
Manganese
Nickel
Phosphorus
Lead
Plant Growth-Promoting Bacteria
Particulate Matter
Sulfur
Selenium
Translocation Factor
Zinc
1
Summary
Worldwide, mining is a source of huge amounts of heavy metal contaminated waste. The
most important environmental health risks associated with unmanaged mine waste are
caused by the formation of acid mine drainage and the spread of metals into the
environment through aeolian dispersion, surface runoff and leaching. Heavy metal pollution
and acid mine drainage lead to a decrease of biodiversity, and an increase in death and
disease of vegetation, wildlife and humans for many miles around the polluted area.
A possible remediation technique is phytostabilization. A long-term vegetation cover would
decrease the spread of dust and heavy metals through wind and water erosion, and decrease
the leaching of heavy metals and acid mine drainage into the groundwater. Vegetation,
possibly aided by microbiota or soil amendments, promotes soil development, nutrient
recycling and the development of microbial communities. It is a non-invasive, cost efficient
method to reclaim mine waste areas and reduce risks of heavy metal pollution and acid mine
drainage.
Revegetation has been successful in nutrient-poor heavy metal polluted mine waste areas
both in temperate and arid climates. Mycorrhiza and plant growth-promoting bacteria have
been found, in some studies, to aid plants in overcoming metal toxicity and decrease
leaching and uptake of heavy metals into the plant. However, not all studies have found this
effect. Mycorrhiza and growth-promoting bacteria can also decrease the amount of soil
amendments needed, lowering costs.
Very few studies on the long-term effects of revegetation on risk reduction have been
performed. For instance, the decrease of aeolian dispersion and surface runoff due to
revegetation has not been studied, and only few have studied the effects on leaching. These
studies, possibly due to large differences in experimental design, show conflicting results. It is
unclear what influence revegetation has on heavy metal leaching. Metal mobility is influenced
by many conflicting processes driven by the plant, bacteria, mycorrhiza and soil status. Most
of these processes only influence the rhizosphere and the top soil layer and are not expected
to influence metal mobility on a larger scale. Acidification due to vegetation does not seem
to be a problem, especially when the right plants are selected. The few studies done show
only a very slight decrease in pH levels over time, or an increase under the influence of soil
amendments and vegetation. Many plant species do not accumulate heavy metals into their
above-ground biomass; these would not pose a risk to foraging animals. In the selection of
plants, and after plant establishment, attention should be paid to metal content in leaves and
shoots. These should not exceed animal toxicity levels to prevent entry into the food-chain. In
order to obtain the best results, a small field study can be performed to select plants with a
low metal accumulation and a high tolerance for the local tailing conditions. This ensures
higher plant survival rates after planting. After plant establishment, long-term monitoring of
the site is necessary.
Phytostabilization has great potential as a cost-efficient and non-invasive method for the
reclamation of mine waste areas, but more research is needed on the effectiveness of
revegetation on risk reduction and to determine which processes mostly contribute to
positive effects.
2
Chapter 1.
Introduction
Mining generates a very large amount of waste. The EPA Toxic Release Inventory reported
that in 2000 the hard rock mining industry was the largest producer of toxic waste, releasing
1.5 million metric tons, or 47% of the total waste produced by the United States industry 1.
Due to an ever-increasing demand for metals, it has become economically feasible to mine
metals when the ore content of rock is as low as 0.01%, such as for gold 2. In this case, for
each ton of metal, 10.000 tons of waste is generated.
The waste is a source of metal leachates and acid mine drainage ultimately leading to a risk
to human health, agricultural activities and ecosystems all over the world. Mine waste areas
are usually characterised by hostile conditions unfit for vegetation due to a lack of essential
nutrients, drought, extreme pH values, a severely stressed or lack of heterotrophic microbial
community and toxic levels of heavy metals 3-8. In a semiarid climate an additional problem is
salinization; salinity builds up as evapotranspiration exceeds infiltration of water 9. High levels
of metals can also adversely influence the number, diversity and activity of soil organisms,
inhibiting the decomposition of organic matter and the nitrogen mineralization process 5.
Metal pollution poses a persisting problem, as metals cannot be destroyed but only
transformed from one oxidation stage or organic complex to another 10. Through surface
runoff, eolian (wind) dispersion and leaching, heavy metals can spread for tens of kilometres
in the environment 11-14. Surface runoff and leaching may pollute the groundwater 15, while
the dust spread by the wind settles on vegetation, including agricultural crops, where they
enter the food-chain when consumed 11. Both acidity and heavy metals can have a severe
impact on ecological 16, 17and human health 18.
Traditional methods to treat mine waste are soil washing, capping, storing the waste in a dam
or in exhausted open pit mines 9, 10. These techniques have drawbacks. For instance, thermal
treatment and soil washing generate high amounts of additional wastes that require disposal
and produce soils that are unsuitable for vegetation 19. In addition, these methods are
generally very expensive 9, 10. In 2000 a total of 156 mining sites in the US were identified,
with cleanup costs estimated between $7 billion and $24 billion 1.
3
A possible non-invasive remediation measure to improve mine waste and other polluted
areas, is phytoremediation. Phytoremediation is the use of plants and associated
microorganisms to remove, immobilise or degrade harmful environmental contaminants.
Phytoremediation is the overarching term for all plant based techniques like phytoextraction,
phytostabilization, phytovolatization and biofortification 19. These low-cost remediation
technologies are considered to be suitable for application to former mining sites. Due to the
huge areas to be treated, traditional methods will be economically unattractive 7.
Phytoremediation works in situ, which contributes to its cost-effectiveness and aims to
reduce exposure of humans, wildlife, and the environment 10.
Phytoextraction is a technique by which plants are used to remove metals from the soil by
storing them into their biomass. The plants are harvested and either disposed in a controlled
way, or the absorbed metals are recovered in a process called phytomining. Phytoextraction
has important setbacks. For instance, there are a restricted number of target metals that can
be extracted. Most tailings are polluted with several heavy metals, while most
hyperaccumulating plants only take up one. The clean-up zone is limited by root depth.
Underneath this zone the metals will stay in place and be susceptible to leaching. In addition,
the extraction rate is low; accumulation levels should be around 0.01 – 1% of plant biomass.
For example in a greenhouse experiment, the hyperaccumulator T. caerulescens removed
Cadmium (Cd) and Zinc (Zn) at a rate of 250mg and 8000mg per kg plant biomass
respectively. The mine spoils were contaminated with 58mg per kg Cd and 3300mg per kg
Zn. Due to the low biomass production of T. caerulescens 100 – 1200 cropping cycles would
be needed to remove Cd, and 200 – 600 cycles to remove Zn 20. For heavily polluted soil like
mine tailings, it could take many decades to decrease pollution levels below intervention
levels 21. Furthermore, the risk of heavy metals entering the food-chain due to herbivory is
high.
Phytovolatization makes use of plant-microbial systems that transform soil contaminants into
volatile compounds that disperse to the atmosphere. The number of metals that can be
volatized by known plant-microbial systems are limited to Mercury (Hg), Arsenic (As) and
Selenium (Se). Phytovolatization also has drawbacks of a long clean-up time and limitations
of root depth, with the added important factor that there is no control on the destination of
the compounds 22.
A more successful technique for mine tailings is phytostabilization. Plants and their
associated microbiota, possibly in combination with soil amendments, are used in situ (nonexcavated soil) to form a protective cap, which immobilizes metals within the tailings. This
method does not remove contaminants from the soil, but focuses on reducing the mobility,
ecotoxicity and spread of metals in the environment and food-chains 22-24.
Many studies have been done on phytostabilization and the most suitable plants and
amendments, but very little is known about the effectiveness of this technique to reduce or
remove risks for human and ecological health. The focus of this paper will be on the
efficiency of phytostabilization as a remediation technique to reduce risks to human and
environmental health caused by heavy metal pollution and acid mine drainage in and around
mine waste areas. For this purpose, the extent to which phytostabilization is effective in
decreasing the spread of heavy metals into the environment through leaching, surface runoff
4
and wind dispersion will be studied. Using this information, it was investigated under which
circumstances phytostabilization is most successful for risk reduction.
To answer these questions the environmental health risks at mining sites will be explained
first. Afterwards, the mechanism of phytostabilization will be discussed, as well as which
processes influence risk-causing dispersion routes around mine waste sites. The long and
short-term effects of revegetation of mining sites will show the influence of phytostabilization
on risk reduction over time.
Chapter 2.
Risks associated with mining sites
Prior to mining the target ore, the top layer of the soil is removed. This so-called overburden
usually does not contain sufficient ore to be feasibly mined. The underlying soil-partition that
contains the ore is taken, pulverised and treated until the target element is removed. To
extract metals from soil substances are often used to dissolve and separate these metals. For
example, Gold (Au) can be extracted by Hg 11 or, the current standard, Cyanide 25, 26. The
resulting waste, called tailings, is a combination of fine-grained soil (typically silt-sized, in the
range of 0.001 to 0.6 mm) and any remaining process water. Tailings may contain substantial
amounts of added compounds used in the extraction process 25.
5
Figure 1. Metal extraction process. Adapted from Dirk de Kramer, Witteveen+Bos, 2011.
Tailings create a hostile environment that inhibit plant growth. Due to its fine texture it
usually has a very low water retaining capacity. Furthermore, tailings are generally extremely
poor in organic carbon content and nutritive capacity 5-7, 27
The physical and chemical characteristics of the tailings vary with the nature of the ore 12.
Most mine-waste contains several different heavy metals 22, 28-30. Modern tailings have a much
lower metal concentration than historic tailings due to improved extraction methods 2, 23.
Former mine sites are frequently sources of pollution many years or even decades after
closure 11, 33. It is this pollution, and the environmental health risks associated with it, that will
be treated in this report,
Mining related pollution
Acidification and Acid Mine Drainage
Acid Mine Drainage (AMD) is the largest source of environmental problems caused by the
mining industry. Acid mine drainage is the result of tailings and overburden being exposed to
air and water. The oxidation of pyrite and other sulphide minerals in the presence of oxidising
bacteria results in the production of acids 34. This oxidation process occurs slowly in all soils.
Tailings and overburden, with their small grain size and thus greater surface exposure, are
more prone to generating AMD. The oxidation process is accelerated beyond the natural
buffering capacity of the host rock and water resources 16. The acid-neutralizing capacity of
soil and water is dependent on its carbonate (CaCO3) content as the carbonate neutralizes
acids 34, 35.
AMD has a very large impact; it is created on a large scale, and due to its mobile nature it can
spread easily through waterways into the environment 11, 16, 27, 33-36.
Mine tailings range from highly acidic (pH 2) to alkaline (pH 9), depending on the carbonate
content and acid-generating potential of the tailings 23. The majority of mining waste areas
are acidic, but in bauxite mining the waste (red mud) is alkaline 37. Sometimes lime is added
during the refinery process to neutralize acids 27.
6
Heavy metal and metalloid pollution
Metals and metalloids occur naturally in the environment. Metalloids are chemical elements
with properties that are in-between or a mixture of those of metals and non-metals. A
metalloid that is found in high concentrations at many mine wastes is As 12, 31, 32. In this report,
the term heavy metals will be used to refer to both metals and metalloids. Some of the
metals like Copper (Cu), Zn, and iron (Fe) are essential nutrients for organisms. Even essential
metals are toxic in excess amounts 38.
There are two processes that determine the release of metals into the environment. Firstly,
there is the natural process of weathering that is accelerated by the larger contact area of the
pulverised waste, liberating metals by oxidation of the sulphide mineralogy 2. The second
process is under the influence of acid mine drainage. Water with a low pH value easily
dissolves naturally occurring heavy metals from the tailings and overburden dumps that
would normally be immobile, such as Cu, Aluminium (Al), Cd, As, Lead (Pb) and Hg 39. This
leads to acid, heavy metal polluted water 16.
Salinity
In semiarid and arid climates soils are often saline. The salt originates from saline rainfall,
groundwater, unweathered minerals and fossil salts 9. Seasonal accumulation of salts occurs
when evaporation rates exceed rainfall 9, 23. When the water table is between 2 to 3 meters
from the surface of the soil an upward migration of salts occurs via capillary forces. In some
circumstances a salt crust forms on the surface 40. Alkalinity is usually combined with salinity,
because of CaCO3 enrichment in the top soil layer 41. These salts can contain metals at levels
exceeding 200-fold the levels measured in tailings 40.
Dispersion and exposure pathways
Surface runoff
Surface runoff, also called water erosion, is especially important in temperate and semiarid
climates where sudden heavy rainfall episodes occur 7, 40, 42. These concentrated surface flows
can erode the tailings and transport severely polluted sludge for many miles downstream to
residential, agricultural or environmentally sensitive areas 13, 40.
Leaching
Another water driven transport route is leaching. Precipitation percolates through the soil
and finally reaches the groundwater. Due to the acidification in most mine waste areas the
leachate can be highly acidic and have a very high metal content 43. This contaminated
groundwater can reach the surface in springs or seeps, which are not usually diluted by fresh
flowing water 44. This can lead to very high metal concentrations, which may severely impact
plant and wildlife 17.
Aeolian dispersion
Due to the lack of vegetation, aeolian dispersion (by wind) is another important route by
which heavy metals can spread through the environment. The fine particles dispersed from
tailings are in themselves already harmful, but additionally the metal content in this dust can
cause health risks.
Meza-Figueroa et al showed that the salts formed on the soil surface, which can contain
extremely high concentrations of metals, are also susceptible to wind dispersion 40.
7
Unconfined tailings, especially in a dry, windy climate, can be an important source of air
pollution for surrounding residential areas where particles can be inhaled or deposited 44-48.
A study of the source and composition of particulate matter around a mine waste area in
Mexico, found that for particles ≤ 2.5 μm (PM2.5) only 1-6% of the overall mass originated
from mine waste, for ≤ 10 μm (PM10) this was 4-39%. On the other hand, for PM2.5 40% of
Pb and 63% of Zn originated from tailings, while for PM10 this was respectively 88% and 97%
49
. This shows that dust from mine waste areas is a highly relevant source of heavy metal
pollution through eolian dispersion.
The most relevant exposure route is dependent on the climate. In temperate regions,
leaching, AMD and in a lesser extent surface runoff will be of greater importance due to
higher annual precipitation 23, 40. In semiarid and arid climates, dispersion by wind will form an
important source of pollution, along with surface runoff in the rain season. Tailings are
usually dry due to their low water retaining capacity. Even in temperate regions the tailings
can be a desert-like environment.
Risk receptors
The particulate matter, AMD and heavy metals originating from mine waste areas cause risks
to humans, wildlife and the environment in general. The toxicity of heavy metals depends on
many factors, including the bioavailability, the target species, the route of uptake and the
concentration in which it is ingested 38, 50.
Risks to human health
In an acute response, increased levels of fine particulate matter can cause illness or death to
people with a respiratory or heart disease or a decreased lung function. Chronic exposure can
increase the risk of lung cancer, heart disease and respiratory diseases 38.
Humans can be exposed to heavy metals via the food-chain, polluted drinking water, dermal
contact, and inhalation or ingestion of dust and soil 38, 48. Health effects of heavy metal
contaminations include cardiovascular disease, developmental and reproductive toxicity,
neurological damage and cancer. Even at extremely low concentrations effects can occur 18, 38,
44
. Young children are most susceptible to heavy metal poisoning as they have a higher
absorption rate and metal sensitivity than adults. Furthermore, their relative intake is higher
due to a higher amount of soil ingestion by increased hand-mouth contact 38.
Some plants adapted to semiarid environments have trichomes and glands that capture the
heavy metal-rich dust from tailings. The captured dust cannot be completely removed by
washing, even after thorough cleaning in laboratories 41. This implies an important effect on
human health by the consumption of dust-containing crops grown in the surroundings of
mine waste dumps and tailings 11.
Risks to the environment
Acid mine drainage has severe detrimental effects on aquatic life. Receiving waters may have
a pH as low as 2.0 to 4.5. Such levels cause death due to hypoxia in fish, and are lethal to
most forms of aquatic life 51. Reports show a complete loss of fish in 90% of streams with
waters with a pH of 4.5.
Streams affected by acid mine drainage are poor in species number and abundance 16.
For fish and benthic organisms, the most toxic elements are Zn and Cd. Even when these
metals are present in surface waters in low concentrations they result in acute toxicity 16, 51.
8
Levels up to 310,000 µg/L of Zn and 370 µg/L of Cd have been found in the river passing a
tailings area in Breckenridge, U.S. The pollution in the upper part of this river prevents fish
survival, and severely restricts the diversity and abundance of benthic invertebrate organisms
17
.
High concentrations of metals in soil can cause phytotoxicity and death in plants, and a
decrease or absence of a microbial community5, 10, 22.
Heavy metal pollution and AMD leads to a decrease of biodiversity and an increase in death
and disease of vegetation and wildlife in the vicinity of severely polluted soils and water
bodies around unconfined mine tailings.
Chapter 3.
Plant - Environment interactions
Revegetation of bare mine waste areas is cost effective, and less disruptive to soil and the
natural landscape than other remediation methods. A vegetative cap is formed that not only
improves the visual impact, but also alters processes that can influence risks associated with
mining sites.
Phytostabilization can influence environmental health risks at mining sites via several
mechanisms; by altering the water flux, adding organic matter to the soil, controlling erosion,
and changing the metal speciation and mobility 5, 9, 23, 43. Revegetation is aided by inorganic
and/or organic soil amendments 41, and by a healthy microbial community 5, 9, 23, 43. Speciation
of the metals determines the mobility, bioavailability and toxicicity of metals 38, 50. A metal is
bioavailable when it is able to interact with biological organisms, including humans 22.
Phytostabilization can increase and decreases metal mobility via many processes 22, 43, 52, as
can be seen in figure 2.
Plant – physical environment interaction
Erosion
Roots stabilise the soil, reducing soil dispersion by wind and water erosion 5, 9, 23. A dense
canopy protects the soil surface from rain impact, contributing to the decrease of erosion.
Water flux
Phytostabilization alters the water flux. Water evaporates from the canopy, decreasing the
amount of precipitation reaching the soil. In addition, root uptake and transpiration reduce
the water flux through soil, decreasing leaching. Of the global average rainfall, around 57% is
returned to the atmosphere directly from the soil surface after rainfall and by
evapotranspiration from vegetation. This represents a significant reduction in the volume of
9
leaching. In arid regions evapotranspiration could eliminate drainage and thus decrease AMD
and the dispersion of metals 43.
By improving the soil structure the water holding capacity increases, contributing to a
decrease in leaching 6, 53.
However, roots may create macropores into the soil, which facilitates rapid transport of
contaminants to the groundwater 22, 54.
Metal mobility and bioavailability
Organic matter and pH are the most important factors that determine metal mobility in soil
55, 56
. Generally, a low pH increases metal solubility 39, even though an increase in As solubility
upon alkalinisation has also been reported 57.
Organic matter enters the rhizosphere via root exudates, decaying tissue and fall-off. This
leads to a healthier soil which is more suitable for vegetation and microbiota 58. Metals bind
effectively to organic matter 22, 43; this sorption decreases metal mobility and bioavailability.
Part of the organic matter is soluble, called dissolved organic carbon (DOC). DOC can
increase the dissolved fraction of the metal, leading to an increase of leaching or uptake by
plants 11, 41, 56, 59. This would mean that as revegetation progresses and more organic matter
enters the substrate, metals can become more mobile with time.
10
Figure 2. Plant – Environment interactions and processes important for phytostabilization
1. Evaporation from canopy reduces water flux. 2. Plant transpiration reduces water flux. 3. Roots
stabilize the soil.
4. Fall-off increases organic matter in soil. 5. DOC can increase leaching. 6. Roots act as a metal sink 7.
Root exudates influence metal mobility and availability. 8. Bacterial colonies influence metal mobility
and availability.
9. Mycorrhiza influence metal mobility and availability. 10. Anaerobe vs Aerobe conditions. 11.
Macropores could increase leaching.
The rhizosphere (soil-root interface) influences the speciation and thus mobility and
bioavailability of metals. For instance, root exudates may acidify the rhizosphere leading to an
increase in metal solubility. Differences between the pH of the rhizosphere and the soil can
cause processes of adsorption, desorption, precipitation or solubilisation of metals 5, 60. Root
exudates may acidify or alkalize the rhizosphere. Values of up to pH 2 have been found 61. A
study on suitable plants for revegetation of mine spoils found that three of the four
examined species strongly alkalized the rhizosphere and even the bulk soil 62. Exudates can
also render heavy metals unavailable for plant uptake, reducing phytotoxicity 22.
Metals can adhere to root surfaces 23, and therefore roots can act as a metal sink by
absorbing and accumulating metals 5. It is very important that the plants do not accumulate
metals in their above-ground biomass, as contaminants could enter the food-chain.
11
Roots help maintain an aerobic environment; soil aeration is improved by extracting moisture
and forming continuous channels for drainage and air exchange. An aerobic environment
helps to prevent the formation of reduced metal species that are often more toxic and more
mobile than oxidized species. However, increased metabolic activity can result in anaerobic
conditions if more oxygen is consumed than can be re-supplied 22. Beside as a result from
plant productivity, anaerobic conditions can also occur due to the tailings being waterlogged.
Plant – microbe interaction
Microflora in the rhizosphere also influences the mobility and bioavailability of metals. The
two most important types of microflora with regard to phytostabilization are mycorrhizal
fungi and plant growth-promoting bacteria. The success of phytostabilization depends upon
a plant's ability to tolerate high concentrations of metals and extreme pH values. Under
growing conditions with high levels of metals most plants synthesize stress ethylene and
have severe shortage of Fe 63. A healthy soil microbial community can be beneficial for the
revegetation process and assist plants in overcoming phytotoxicity 8, 31, 63
Mycorrhizal fungi
Mycorrhizal associations exist in several forms. One of the most widespread associations
exists between arbuscular mycorrhizas and the roots of terrestrial plant species 64.
Mycorrhizal fungi can facilitate the absorption of water and nutrients by increasing the
surface area of plant root systems 6, 65. They can also protect against toxicity of heavy metals 6,
10, 22, 64
. Most studies report that mycorrhizal fungi can decrease the leaching and uptake of
heavy metals in plants 65-68. However, other researchers do not find this effect 66, 67.
Contrasting evidence of studies on the effect of plant inoculation with mycorrhizal fungi
suggests that the effect is highly dependent on the plant species, the kind of metal and the
species of mycorrhiza 10, 65-67. This makes it very difficult to predict the effect of inoculation on
phytostabilization success.
Plant growth-promoting bacteria
Plant growth-promoting bacteria (PGPB) can increase plant biomass, root and shoot length
by reducing stress ethylene; solubilise Phosphorus (P), Nickel (Ni), Potassium (K) and other
essential nutrients; fixate nitrogen; and aid in seed production and germination 63, 69, 70. PGPB
can also increase plant tolerance for flooding 71, salt stress 72, and water deprivation 73. This
can be very important in tailing areas which are often saline and either waterlogged or
extremely dry. PGPB can also protect the plants from insects, and fungal, bacterial and viral
diseases 69. Mechanisms for heavy metal resistance in bacteria are exclusion, sequestration or
metabolism into a less toxic speciation 74. This detoxification process enhances tolerance of
plants to stress caused by exposure to heavy metals 10, 69. This would improve the survival and
growth rate of plants, leading to a faster revegetation and bigger canopy. As for mycorrhizal
fungi, PGPB inoculation can either elevate or reduce the uptake of heavy metals 10, 67-69.
A microbial community is fundamental for the biochemical cycling and the decomposition of
organic matter 41, 67. Studies show that mine waste areas have a severely stressed, or even
absent microbial community 5, 6, 31. To increase chances of phytostabilization success, seeds
can be inoculated with microbiota 58, 70.
12
Results from studies on the influence of both mycorrhizal fungi and PGPB on leaching and
plant heavy metal uptake show mixed results. Field studies are necessary to determine the
microbial species with the greatest potential for each mining site.
Soil amendments
The direct establishment of plants on mine tailings sites almost always requires inputs in
terms of compost or nutrient amendments 23, 75. These amendments increase the survival rate
and colonization of vegetation 76. In addition to facilitating plant establishment and growth
by improving the soil structure, water retaining capacity and pH value of mine waste, soil
amendments can also directly decrease the risks of mine waste dumps by immobilizing
metals 24, 52.
Soil amendments should be cheap, non-toxic to plants, widely available, easy to apply, and
safe for workers to handle 10. By-products of a production process, or amendments that have
little to no economic value such as oyster shells, manure or biosolids are preferred 8, 77. The
choice of soil amendment will sometimes be limited by its availability in sufficient
quantities41.
Organic amendments
Organic amendments are usually meant to add essential nutrients and organic matter to the
soil, inoculate it with microorganisms and mitigate metal toxicity 4, 41.
Commonly used examples of organic amendments are biosolids (treated human waste) 65, 7779
, compost 53, 62, 66, 67 and woodchips or other plant residues 5. Compost can improve the
water holding capacity, which can decrease the amount of AMD leaching to the groundwater
53
. Compost also provides a direct and slow release source of nutrients 5
Inorganic amendments
Inorganic amendments are generally used to improve either physical characteristics like soil
structure, or chemical characteristics such as pH and metal mobility 41.
When the limit for the buffering capacity of CaCO3 occurring naturally in the soil has been
surpassed, mine tailings acidify. Many mine waste areas have extremely low pH levels due to
the resulting acid mine drainage 11, 33. This can be countered by liming 52, or addition of other
alkaline materials such as cyclonic ashes 52, 80, 81. Since metal mobility greatly increases under
low pH circumstances, liming favours sorption to soil, leading to a decrease in phytotoxicity
80
. Some studies show that liming also inhibits translocation of metals, particularly Pb, from
root to shoot 82. Adding alkaline materials is only a temporary solution. With time, the soil
starts to acidify again if the buffering capacity of the soil has been surpassed 23.
Other inorganic amendments are for instance fly-ashes 80, steelshots 53, 83, inorganic fertilizers
or rubble 41. Fly-ashes and steel shots corroding in soil can supplement essential plant
elements such as K, Calcium (Ca), Magnesium (Mg), Sulfur (S) and Boron (B) 80, 83. Rubble
increases the soil structure, and can provide some nutrients. Before application the rubble
should be tested on metal content and AMD generating capacity 41.
A combination of organic and inorganic amendments generally leads to the highest success
rate of revegetation experiments 53, 55, 83. The downside is the higher cost. It depends on the
concentration of heavy metals on the site if inorganic amendments to decrease bioavailability
are essential. Litter degradation, aided by microbiota, will in later stages of revegetation lead
to a self sustainable ecosystem without the need of further amendments 58.
13
Negative effects
Most amendments have undesirable side effects. Some amendments may immobilize
essential nutrients together with the heavy metals 24. Metal mobility can be increased by the
utilization of biosolids 41, 79. The use of compost can lead to an increase 53 or a decrease of
metal mobility 83. Part of the added organic matter is soluble, leading to increased leaching of
metals or uptake by plants 56, 56, 59. Biosolids and fly-ashes also contain high concentrations of
salt 41, 80.
Influence of microbiota on soil amendment use
Many mine waste areas require extensive amounts of soil amendments to make plant growth
possible. For successful revegetation, mine tailings may need up to 15% soil amendments
mixed in 62. These soil amendments are often the most expensive part of phytostabilization 23.
PGPB and mycorrhizal fungi can minimize the need for amendments, decreasing the cost of
phytostabilization 65, 70, 84. Grandlic et al compared growth of PGPB and non-PGPB inoculated
plants on extremely acidic heavy metal polluted tailings 84. The results showed a decrease in
the amount of compost amendment needed for normal plant growth, and an increase in
biomass for inoculated plants of up to 400%. This effect was greater in soils with no added
amendments, indicating that especially for situations where no or very little compost is
available the presence of PGPB or mycorrhiza can make a big difference in phytostabilization
success.
Chapter 4.
Plant – Community interactions on
risk reduction
Characteristics of tailing-resistant vegetation
There are several important characteristics that make plants suitable for the revegetation of
mine waste areas. Plants used for the vegetation of tailings should be native. Furthermore,
they should be resistant to drought and salt stress, high temperatures, extreme pH values
and low nutrient availability 75.
Local species
Species should be native, common to the region or at least non-invasive. The use of
potentially invasive species may result in a decrease of regional plant diversity. Revegetation
14
with native or at least non-invasive plants is especially important for tailings situated in
protected and environmentally fragile areas of the world 28, 76, 85. Native plants are also well
adjusted to the climate, leading to a higher survival rate.
Metal and salt tolerance
Metal tolerance in plants is very important for the revegetation success of tailings 23. Most
species are only tolerant for one metal, usually only for the metals that occur in the soil in
which they naturally grow 86, 86. This is another advantage of using local plants, especially
plants already adapted to the harsh conditions on the mine tailings.
As many tailings are saline, it can be necessary to use halophytes (salt tolerant species).
Legumes
Tailings are usually extremely poor in nutrients, making revegetation difficult. Plants which
could greatly improve the soil characteristics without the need for amendments are legumes.
Legumes are plants that fixate and accumulate nitrogen in a mineralized form in symbiosis
with bacteria. This characteristic makes legumes well suited to grow and survive in low
nutrient conditions 87. Growing legumes on tailings will improve the rate of healthy soil
culture formation 88. When legume species are mixed with non-legume species, the biomass
of the latter also increases 89.
Characteristics of risk reducing vegetation
The most important factors for the environmental health risk reduction of mine waste areas
are the prevention of heavy metals entering the food-chain, and to stop their spreading into
the environment by leaching, surface runoff and wind dispersion. Remediated mine waste
areas should preferably form a self-sustainable vegetation cover. For risk mitigation strategy,
the vegetative cap should stay in place indefinitely, unless a different solution is planned for
the future 23.
Food-chain
An important aspect of phytostabilization, as opposed to phytoextraction, is that metals
should not accumulate in the above-ground biomass. Shoot accumulation would facilitate
entry into the food-chain by foraging animals 11, 13. Additionally, through litter fall, metals
would spread into the environment and accumulate on the soil surface 28. In addition to
being spread via biomass accumulation, the metals can provoke a direct toxic effect to plants
and foraging animals. Table 1 shows guidelines for metal toxicity limits for soil, leaf tissue and
animals 9. The domestic animal concentrations are based on above-ground metal
accumulation, since foragers as cattle and wildlife can ingest these plants.
For the selection of plants for tailing revegetation purposes, two factors are important. The
Bioconcentration Factor (BF), also known as the accumulation Factor (AF), is defined as the
ratio of metal in the shoot tissue to metal in the contaminated medium. The translocation
Factor (TF) is the shoot-to-root ratio of the metal concentration 90. To prevent entry of metals
into the food-chain both these values should be below one. Furthermore, shoot metal
concentrations should not exceed the domestic animal toxicity limits 23, 75. There is a large
difference in metal uptake between plant species 91; as explained above, plant roots should
exclude metals to prevent accumulation into the above-ground biomass.
Table 1. Metal toxicity limits (mg/kg)
15
Toxicity index
As
Cd
Cu
Mn
Soil toxicity levels for plants¹
15
3
200
3000
90
5-20
5-30
220
40
4001000
2000
10-100
100500
30-100
100
100
Plant leaf tissue toxicity limits²
Domestic animal toxicity
30
limits³
Table by Mendez et Maier (2008) 9
10
Ni
Pb
Zn
400
100400
500
Leaching
In high-rainfall periods, a vegetative cap will decrease the amount of leaching but will
probably not eliminate it. When rainfall is greater than evapotranspiration, drainage is
inevitable. A model estimating drainage of a contaminated site with and without a vegetation
of poplar trees showed that, in the first 4-5 years of establishment, drainage will occur
throughout the year 43. After this period, drainage will only occur during the winter months,
even in temperate climates 43. Vegetative caps may eliminate drainage during low-rainfall
periods and decrease the concentration of heavy metals in the leachate. Due to root
exudates, the long-term influence of vegetation can result in soil acidification. This would
cause an increase in metal mobility 80.
Shrubs and trees
Trees are an essential component of a phytomanagement strategy. They are long-lived
organisms that will stabilise the environment for a long time. Deep-rooting species of shrubs
and trees, preferably evergreens, are most effective for the reduction of leaching. They can
access water from a greater depth, forming a dry buffer zone that can absorb water following
a heavy rainfall event 28. This also decreases the salinization of soil in semi-arid climates by
lowering a saline water table, which decreases salt toxicity to plants 22. High
evapotranspiration rates decrease leaching, deep roots decrease surface runoff (water
erosion), and the canopy can protect the surface from rain impact 9. During periods of
drought, deep-rooting species will have a higher survival rate, as they will continue to have
access to water for a longer period of time 43.
Grasses
Grasses are quick growing organisms with extensive rooting systems, making them very
suited to provide a quick ground cover which reduces wind erosion. They can even be used
as a temporary solution to decrease aeolian dispersion until the desired climax vegetation
can be established 22, 92. Grasses are often very tolerant to extreme conditions, making them
suitable for revegetation of mine waste areas.
Perennial species
To constantly reduce wind and water erosion and leaching, it is important to have a
vegetative cover present throughout the year. For this reason it is better to use long-lived
perennial species for revegetation purposes.
Climate dependence
For arid climates plants should be drought and salt tolerant 8, 9, 22, 23, 85. Salinization can
successfully be treated with deep-rooting, high water-use tree species 93.
16
Tailings can be either extremely dry due to its low water retaining capacity, or water logged.
In water logged circumstances, tailings require plants that are adapted to slightly anaerobic
conditions 23. For temperate regions, AMD and leaching are the main problems 23. To reduce
these problems, a vegetation with high evapotranspiration and a wide canopy would be most
effective.
Plant selection
Since no contaminated sites are alike, choosing the most suitable species could benefit from
a short planting trial that tests several varieties on a small area of the site 22. Local plants can
be germinated and grown on tailings, or tailings mixed with a 4:1 ratio of topsoil. After
several months the best performing species with low metal uptake can be transferred to the
tailings for phytostabilization. This could reduce costs and decrease the time needed for
successful plant establishment.
A mix of different species and kinds of vegetation will ensure that a plague or unfavourable
climatic condition will not affect the entire population 43, 75. The result is an area which
provides food and shelter for animals and is more aesthetically attractive as well.
An efficient method for the selection of suitable plants might be to sample vegetation that is
already growing on the mine waste area. These plants are obviously well adapted to the
climate and harsh conditions. In addition, they are readily available. Samples of plants
growing in the mine waste area should be examined on metal accumulation, and metal
translocation to the shoots. Several studies showed that the metal uptake by spontaneous
vegetation in acidic mine tailings was low 7, 12, 42. When comparing the metal concentrations
of plants collected in the field with plants grown in a greenhouse, it was found that wild
plants had metal concentrations an order of magnitude less 94. A study found that Lygeum
spartum, one of the plants found growing wild on mine tailings with a low metal uptake,
performed very badly in a pot experiment using acidic tailings 94. The plant did not tolerate
these conditions, and had a high metal uptake. The authors hypothesized that by gradually
invading from the edges where the conditions were more favourable, an adaption process
had taken place 7, 94. This also implies that making use of plants already growing on mine
tailings, adapted to the harsh conditions, could be a successful strategy for revegetation. For
a more rapid and successful development of a self-sustaining ecosystem, additional species
might be considered in case these are not present, for instance legumes and deep rooting
trees.
Genetic engineering
Some research has been done on the potential of genetic engineering to improve the success
of phytoremediation techniques. Most of these experiments have been performed with the
intention of increasing the metal uptake of plants for improving phytoextraction, but
attention is also extended to factors important for phytostabilization. For instance, enhancing
the metal tolerance of plants and increasing the growth and biomass production of
transgenic plants. Some studies have shown promising results, but no practical applications
of transgenic plants have been reported at the time of writing. A possible risk involved with
genetic modification is biological transformation of metals into a more bioavailable
speciation 10.
17
Crops of economic value
A way to decrease the costs of phytostabilization, and to profit from land which has to
remain vegetated, is to grow crops with an economic value. Examples are the production of
biofuel, timber, cut flowers, cotton or stock fodder 22, 24, 75. All products, and in particular stock
fodder, would have to be closely monitored to prevent the use of contaminated products 5.
Many of these products are annual plants, and by producing and harvesting these on the
mine tailings the risks of erosion and leaching will increase. After harvesting,
evapotranspiration will also decrease, and potentially leaching will increase. Additionally,
Gonzalez-Sangregorio et al 3, found that harvesting from an area being revegetated led to a
slower improvement of soil quality. A possible solution is to wait several years until a solid
vegetation and root system is established, before starting the periodical harvest of crops for
economic purposes. The timing and nature of the harvest is critical, and at all times there
should remain sufficient perennial plants to control leaching and erosion.
Chapter 5.
Results of revegetation studies
Surprisingly few studies have been performed on the influence of phytostabilization on
contaminant spreading and associated environmental health risk reduction. The focus of
most studies is on plant and microbial processes influencing metal mobility. Extensive
research has been done on the effect of soil amendments, sometimes combined with
vegetation, on pH and the leaching of metals.
Most studies that have been performed on the effect of revegetation are short greenhouse or
lysimeter experiments, ranging from 49 days to several months. At the moment of writing,
only five field experiments of three years or more have been documented. Of these studies,
only one studied metal mobility, but not leaching, after revegetation. None of the studies
looked at effects on erosion.
Plant establishment
To help young plants settle it can be important to irrigate the site during dry periods. A study
where this was not done experienced a high percentage of tree death during an exceptionally
dry period 80, while revegetation was successful for several studies that did use irrigation 6, 58.
There has been one study describing the different phases of ecological succession on mine
restoration. These phases can be described as follows: (1) an initial establishment phase (0–6
years) characterized by nominal spoil changes and survivability of planted trees; (2) a brief
transitional phase (6–12 years) characterized by the increased canopy of the tree plantation,
immigration of animals and gradual changes in soil properties; and (3) enrichment phase (12–
18 years) characterized by rapid development of ecosystem including regeneration capacity
of newly developed soil, nutrient cycles and ecological niche 6. After the immigration of
animals, and shelter and a source of food were established, birds and insects transported
18
seeds from the surrounding biodiversity into the area, leading to an increase in plant
diversity. After 18 years the maximum water holding capacity was increased from 29.80% to
51.20%, and the organic carbon increased from 0.99 g/kg to 145 g/kg. The pH remained
similar, between 6.9 and 7.4. A nutrient rich soil high in nitrogen, P, K and organic carbon was
established, and 350 higher plant species settled into the area.
Surface runoff
No scientific studies on the effect of revegetation on surface runoff have been performed.
Two studies did show that the root biomass of plants growing in tailings is larger compared
to the above-ground biomass of plants not growing in tailings 44, 84. The dry and nutrientpoor conditions prevailing in tailings could force the plant to invest in an extensive root
system. This could have implications for stabilisation success and erosion. A relatively small
above-ground biomass will lead to less organic matter accumulation on the surface, and thus
slow the soil forming process. The smaller canopy will provide less protection against rain
impact, and evapotranspiration will be lower. Deeper roots could cause larger macropores,
which could increase leaching. On the other hand, a solid root system will decrease water and
possibly wind erosion.
Acidification
In theory, revegetation could lead to acidification of soil, increasing metal mobility. Some
studies on this effect have been performed. In an abandoned goldmine contaminated with
As, after five years of revegetation, no effect of trees on As availability or soil pH was
detected. Only in the extreme upper layers a drop of average pH from 7.3 to 6.3 was detected
32
. An eight-year-long field study on highly contaminated soil, where plots were either
planted with just a tree mix, or trees and fly ash, found a decrease under the influence of
vegetation, from an average pH of 7.84 to 7.31 80.
In a study of Cu contaminated soil, which had acidified due to addition of NH4+NO3− fertiliser,
the soil recovered more rapidly in vegetated than unvegetated pots 95. After 18 years of
revegetation, Juwarkar et al found a slight increase in soil pH, from an avarage pH of 6.9 in
year one to 7.2 after 18 years 6.
A 60-day plot experiment added compost and several species of plants to extremely acidic
tailings, with an average pH of 2.5. The compost elevated the pH levels temporarily, but due
to the acid-generating potential of the mine tailings the pH quickly declined again. However,
depending on the amount of compost added, some plants greatly influenced pH levels. At
15% compost addition, some plants dramatically prevented the reduction in pH of the
tailings samples. At 20% compost, where the pH was much higher, the opposite effect was
observed; unplanted controls had higher pH values. The plant species that engaged in
alkalinization at the lower compost levels did not do so at the 20% compost level. The
authors speculate that the initial soil pH determines if plants with the capacity to modify the
pH of their environment will acidify or alkalize. To support this, an indicator assay was
performed. Seedlings were placed between two agarose gel slabs amended with the pH
indicator bromocresol purple. This showed that the plants which performed best in the pot
experiment increased the pH of the tailings 62.
19
It seems that acidification due to revegetation is not a concern. The values found show either
an increase or a very slight decrease in pH values. In some studies the observed drop could
be due to the added soil amendments. As shown above, some plants are able to influence
the pH of their environment. These could prove of great value in the revegetation of acid
mine tailings.
Leaching and metal mobility
In a 49-day greenhouse experiment performed by Banks et al 68, it was found that pots with
vegetation had more leaching of metals than pots with vegetation inoculated with
mycorrhiza. Both had more metal leaching than unvegetated pots. The authors speculated
that this could be due to acidification and complexion of the Zn by root exudates. This
combination would make Zn much more mobile. Macropores created by the roots are
another mechanism that could increase leaching. A lysimeter approach, evaluating during 3
months the effect of compost, expanded clay, vegetation, and mycorrhizal and bacterial
inoculation on leaching of heavy metals from mine waste, found an increase in plant uptake
and metal mobility in all treatments compared to the controls. This is largely due to the
higher plant biomass and higher hydraulic conductivity in the amended plots 67.
Six years after setting up a lysimeter experiment with revegetated mine spoils and soil
amendments, no correlation was found between metal mobility and microbial parameters
and plant species richness 31. This study does not look at leaching, and the focus of the study
is on the soil amendments and not on the effect of vegetation. A year-long second study,
where clean subsoil and contaminated mine tailings were covered with topsoil, showed that
plants reduced the volume of leachate and Pb leaching, but the amount of Zn and Cd
remained at a similar level 96.
Other studies found a decrease in leaching after phytostabilization techniques were applied.
In a 77-day pot experiment with Pb-contaminated soil planted with pine seedlings,
revegetation led to a decrease of Pb solubility of up to 93% in mineral soil 97. A 24-week
greenhouse experiment, where mine tailings were planted with Solanum nigrum and
amended with compost or manure, led to a decrease of up to 80% in Zn leaching. In most
cases the sole establishment of S. nigrum resulted in a significant reduction in leachate
volume 66.
It is unclear whether vegetation increases or decreases leaching in the long term, as results
are inconsistent. Neagoe et al suggested that the increase of metal mobility in their
experiment could be due to soil disturbance induced by planting and application of soil
amendments 67. This implies that the short-term experiments would overestimate heavy
metal leaching. From the performed studies no conclusions can be drawn on the effect of
revegetation on leaching or metal mobility.
Exposure of the food-chain
There are concerns that revegetation could lead to the food-chain being exposed to heavy
metals. Some studies to this effect have been performed, looking at bioavailability or aboveground uptake of metals, and the exposure of grazers.
20
A study in a metal and As contaminated waste area found that coarse and fine roots stored a
significant amount of metals and As, but that the translocation to above-ground tissues was
marginal and thus posed little risk of food web contamination 44. As previously mentioned,
several studies found that the metal uptake by spontaneous vegetation in acidic mine tailings
was generally low 7, 12, 42. This would mean that it is possible to select a vegetation which does
not pose a risk to the food-chain. No research on the influence of revegetation on the
exposure of soil dwelling and root consuming organisms, and the threat to the food-chain,
have been done.
Another study looked at bioavailability. In a 60-day pot experiment, mine tailings were
remediated with plants and compost, leading to a decrease in metal bioavailability 62. This
decrease was probably largely due to compost addition, where metals bind to organic
matter.
Several studies have been done on the risks of herbivory in revegetated tailing areas.
Lottermoser et al 98 warned that abandonment and neglect of rehabilitated areas could lead
to toxicity risks to grazing animals. Furthermore, these areas might be invaded by plant
species that accumulate high levels of metals in the above-ground biomass.
However, a study on a pasture polluted with heavy metal containing sludge from a mine
waste spill found no critical values in the vegetation. Predicted values of daily intake for the
horses grazing this pasture were far below critical values reported to induce toxicity. Also, in
faecal and hair samples no elevated values for metals were found 99. King et al found only
limited insect damage to leaves in the five-year period after trees were established on an
abandoned gold mine 32. In a Manganese (Mn) mine, which was monitored for 18 years,
revegetation led to the immigration of animals and diverse population with no apparent
harmful effects on health or reproduction of animals 6. However, no tests to this effect were
performed.
21
Chapter 6.
Discussion & Conclusion
The most important environmental health risks regarding mining sites are caused by the
formation of acid mine drainage, and the spread of heavy metals into the environment by
aeolian dispersion, surface runoff and leaching. These processes can cause severe risks for
human and environmental health for many kilometres surrounding the mining site. After
revegetation, plants containing high concentrations of metals in their leaves and shoots
which could also cause a health risk when they enter the food-chain.
Processes determining metal mobility
There are many processes of phytostabilization that could alter the generation of acid mine
drainage or the dispersion of metals, some of them conflicting. Macropores and the addition
of dissolved organic carbon to the soil could increase metal mobility or bioavailability.
Bacterial and mycorrhizal processes, and root exudates, can either decrease or increase metal
mobility and availability. Many of these processes only influence metal mobility in the
rhizosphere and the top soil layer. It is unclear to which extent these processes actually
increase leaching on a larger scale. For instance, metals mobilised by a low pH in the
rhizosphere could precipitate again in slightly deeper earth layers where root exudates have
not acidified the soil. Metal mobility would only be increased in the rhizosphere, but not for
the total area.
On the other hand, the bioavailability in this area is important. The rhizosphere is the place
where plants take up nutrients and metals, and the top earth layer is a habitat for soil
dwelling organisms.
Success factors for phytostabilization
Soil amendments and microbiota
There are some important factors for successful revegetation of a mine waste area.
Due to the harsh nutrient-poor conditions in mine tailings, both microbiota and soil
amendments seem to be essential for the establishment of a healthy vegetation. PGPB and
mycorrhiza can increase plant survival and biomass production. In addition to this, some
studies have shown they can significantly decrease the amount of soil amendments needed
for successful revegetation 65, 70, 84. Since soil amendments are the most expensive part of
phytostabilization, and phytostabilization is mainly used as an alternative for expensive cleanup technologies, the possible decrease of necessary amendments can be important for largescale application.
Crops of economic value
22
To increase the vegetation rate, it is recommended not to remove plant material from the site
by grazing or litter removal. This has implications, at least for the first years after the process
is started, for growing crops for financial return. Harvesting should always be done with the
utmost care for growing seasons and quantities. In remote areas attention should be paid
that no illegal harvest or logging activities are performed.
Maximise risk reduction
To minimise the spread of metals into the environment by both wind and water it is
important to establish a quick ground cover. Trees, with their deep root systems, are essential
for long-term erosion prevention, and a thick canopy protects the soil from rain impact. Their
high water-use and the evaporation from the canopy could help decrease leaching.
To prevent leaching and erosion during the winter months, species should mostly be
perennial. Using plants growing on mine tailings could provide a good basis for the
vegetation. Plants collected from mine tailings have a higher survival and a lower metal
accumulation rate than the same species not taken from tailings 94. As it is improbable that
the diversity of species necessary for successful revegetation can be found growing on mine
tailings, a complementary selection of trees, legumes or ground covering grasses, for
instance, should be made.
Finally, due to the diversity in climates and tailing conditions, the selection of species should
be done on a case-by-case basis. For the best results in survival rate, and to make sure that
plants do not exceed animal toxicity limits, a small field study should be done.
Shortcomings of the performed research
Many studies have been performed on the most suitable plants for phytostabilization, micro
processes in the rhizosphere, and the effect of soil amendments. Very few studies have been
performed on the efficiency of phytostabilization on risk reduction; only minor attention has
been paid to the effect of revegetation on leaching and metal mobility, while no studies have
been performed yet on aeolian dispersion and surface runoff. Most studies are short-term,
small-scale, greenhouse or lysimeter experiments. In this short time, roots and shoots will not
have developed to their full potential. Many of these experiments do not use trees, which
with their discussed advantages are theoretically invaluable for phytoremediation.
Meanwhile, research indicates that trees are established only after four to six year, and reach
their potential for the reduction of leaching 6, 43. It was also suggested that the disturbance of
the soil in the process of planting and applying soil amendments could temporarily increase
leaching 67. This implies that the short term studies would overestimate the amount of
leaching. It is possible that, many successful revegetation programs for mine waste areas
have been done worldwide, but these have not been documented in scientific studies.
Results of field studies
Acidification due to vegetation
Concern was raised that vegetation could decrease the pH of soil, which could result in an
increase of metal leaching. Of the studies that looked at pH most find either no influence, or
an increase in pH 6, 58, 62, 95. Only two studies report a slight drop in pH 32, 80. It seems that
revegetation, in some cases aided by the right amendments, does not have to lead to
acidification.
23
Leaching
The studies done on leaching show a larger spread in results. Of the six studies previously
discussed, two found an increase in metal leaching or metal bioavailability. The only two
long-term studies, of one and six years, did not find a clear association, while two short-term
studies showed a significant decrease in leaching. The differences between the untreated and
phytoremediated plots are generally small. These varied findings could be the result of the
big differences in experimental design; the type of soil amendments and microbiota, plant
species, climate, measurement techniques and time all vary between the different studies.
This makes it difficult to compare the results.
Influence of microbiota
Studies show that inoculation of plants with mycorrhiza or plant growth-promoting bacteria
(PGPB) can reduce the amount of metal leaching and the bioavailability. In these studies
some conflicting results have also been found; it is highly dependent on the species of
microbiota and the plant species if there is a reduction or an increase in mobility and
availability. The inoculation with mycorrhiza and/or microbes can reduce leaching to less than
untreated soil 68.
Revegetation risks to the food-chain
Wild plants, not selected for their low metal uptake, could spontaneously settle in the area
and pose a risk to animal health. However, studies showed that the metal uptake of
spontaneous vegetation on tailings was low 7, 12. Evidence suggests that phytostabilization
does not pose a health risk to foraging animals, though long-term monitoring should be
implemented.
When to apply phytostabilization
Revegetation has been successful in different climates and mine waste areas, therefore can
be applied broadly. For temperate regions, where water availability usually is not a factor, it
might be easier to develop and sustain a vegetation sufficient for risk mitigating properties.
Vegetation of temperate regions generally provides a better cover than in arid climates.
Sites should stay vegetated, and harvests should be done carefully and sparingly. This makes
phytoremediation less suitable for high value land or areas with land shortage.
To keep the risks of mining sites under control it is important that the site stays vegetated
indefinitely, or until a different clean-up method is selected. As the establishment of plants
can take up to 6 years, phytostabilization is not suitable as a short-term solution.
Conclusion
Phytostabilization has great potential as a non-invasive cost-efficient technique to reclaim
abandoned mine waste areas, and prevent metals from spreading into the environment
through wind and water erosion. Before phytostabilization can be implemented at a largescale more research should be done in several areas. Most importantly, the long-term fate of
metals, and the influence of phytostabilization on risk reduction. More long-term field studies
on the effect of phytoremediation on leaching, aeolian dispersion and surface runoff are
needed. Another field which should be studied more is the minimal required addition of soil
24
amendments, and the influence of microbiota. Decreasing necessary soil amendments could
significantly reduce the costs, increasing the likelihood of implementation. Sites should be
monitored long-term to ensure an ongoing vegetation community. Plant metal contents
should also be monitored to be below animal toxicity limits, which will prevent severe risks to
human and wildlife.
25
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